In airborne communications applications, communications equipment is used to send and receive communications signals. Typically, antennas are located on the outer fuselage of an aircraft while other communications equipment is located inside the aircraft. An antenna (e.g., a radio antenna or optical aperture) provides a means for the radiation of an electromagnetic communications signal from the aircraft, for the reception of electromagnetic communications signals directed toward the aircraft, or both.
An antenna (or group of antennas) can have both a field of view and a field of regard. Field of view (FOV) is the area which is visible to the antenna at a given instant in time. Field of regard (FOR) is the area over which an antenna can communicate. For example, a steerable directional antenna may have a relatively small FOV (e.g., a narrow beamwidth), but may be pointable in different directions to provide a large FOR. A 100% FOR corresponds to full spherical coverage, or a spherical angle coverage area of 4π steradian.
Depending on where antennas are positioned on an aircraft, differing FOR can be obtained. For example, an antenna positioned on the underside of an aircraft can theoretically provide hemispherical (50%) FOR, in that it could theoretically be able to communicate with any object below the plane of the aircraft. In practice, however, the achieved FOR for such an antenna is typically much less than the full hemisphere. Practical limitations include obstructions, such as other portions of the aircraft structure, difficulty in providing antenna operation at small angles relative to a surface on (or in) which the antenna is mounted, and obstruction by the aircraft due to maneuvering. Accordingly, actual FOR achieved by a single antenna is often substantially less than hemispherical (e.g., 45% or less, or even 25% or less).
Conventionally, to provide an aircraft with an overall 100% FOR, six or more antenna/apertures located strategically on the outer fuselage of the aircraft are used. Unfortunately, each of these antenna/aperture installations affects the aircraft's performance and cost. Positioning of antennas on an airborne platform can be challenging, as the location of the antennas on the fuselage is often constrained by a number of factors. These factors can include aerodynamic considerations, access consideration, cabling considerations, and the like. Accordingly, in many applications it is not possible to provide an adequate number of antennas in adequate positions to provide a full or nearly full FOR in an aircraft communication system.